This application claims the priority of German application 100 18 3107, filed Apr. 13, 2000, the disclosure of which is expressly incorporated by reference herein.
The present invention relates to a plastic fuel tank and to a method for producing the fuel tank.
The plastic fuel tanks used in motor vehicles are presently produced by the blowing method. This method is cost-effective, and materials can be used which provide properties necessary for fuel tanks (high impact strength, low permeability). With fuel tanks produced by the blowing method, however, the mounting of fittings necessary for the fuel tank presents difficulties, because these fittings have to be introduced through a mounting orifice.
To solve this problem, WO 97/03511 proposed to produce the fuel tank by welding together an upper plastic shell having the upper terminating wall and a lower plastic shell forming the bottom. The two plastic shells were produced by the injection-molding method. One disadvantage of these known fuel tanks, however, is that the materials normally used for injection molding do not meet impact strength requirements. Furthermore, gases and vapors may escape through tank walls made from known injection-molding materials. In addition, because of the high pressures occurring during injection molding, the molds for this method have to be made very robust and are therefore very costly.
An object of the present invention is to provide a cost effective plastic fuel tank and a method for producing same. In particular, the method is to be simple to carry out, and the tank according to the invention is to satisfy the requirements as to impact strength and permeability to gases and vapours.
This object has been achieved by providing a plastic fuel tank which consists of two halves which are initially produced in one piece by the blowing method. The two halves are produced in one work step and in one mold. By the blowing method, a plastic casing forming a cavity is produced, which is subsequently severed at points provided for this purpose, for example on its circumference. After the plastic casing has been severed, the necessary recesses can be made and/or fittings, such as electrical connections, a filling orifice, filters, etc., can be installed in the halves without any appreciable outlay and the halves can thereafter be connected to one another again.
The advantage of the fuel tank according to the present invention is that the two halves are produced in one work step and one mold. This has a positive effect, namely a lowering of the production costs and of production time. Moreover, the fittings can be installed in the fuel tank in a simple way, because the two halves, in particular also their sides which face the interior after assembly, are accessible from all sides and installation does not have to take place only through a limited orifice.
Furthermore, by way of the blowing method, plastics can be processed which meet the requirements both as to impact strength and as to barrier properties with respect to gases and vapors. This is possible, in particular, in conjunction with coating methods, such as, for example, fluorination, or with multi-layer plastics which, for example by the coextrusion blowing method, can be shaped into a casing which forms a cavity. By a plurality of layers of plastics being used, the positive properties of different plastics can be combined. The tank wall may thereby be given properties which either cannot be achieved with one raw material alone or, if only one plastic suitable per se were used, would entail excessively high material costs.
The desired property combination of impact strength and barrier properties with respect to gases and vapors can be achieved by a carrier material being selected which has high impact strength, for example a polyethylene with high density (high-density polyethylenexe2x80x94HDPE). A barrier layer, for example composed of polyamide, may then be introduced between this carrier material. The barrier properties of the tank wall can thereby be increased, without the impact strength of the carrier material having to be dispensed with.
Because the barrier layer often consists of a material of a different kind from the carrier material, an adhesion promoter layer is arranged between the carrier material and the barrier layer, so that a tank wall constructed in this way has, for example, the following appearance: carrier materialxe2x80x94adhesion promoterxe2x80x94barrier layerxe2x80x94adhesion promoterxe2x80x94carrier material. The coextrusion blowing method per se is known, so that there is no need here for further detail regarding that method.
The above objects are also achieved by a method in which a tank casing is first produced by the blowing method. The casing is thereafter severed along a line provided for this purpose, for example along its circumference. Fittings can be integrated in a simple way into the separated casing halves then present, because all the regions of the halves are freely accessible. The two halves are subsequently laid one onto the other, specifically in such a way that they enclose a cavity, and are connected to one another again.
In a currently preferred embodiment of the method according to the present invention, the casing produced by the blowing method has a U-shaped protuberance along its circumference. To sever the two casing halves, then, at least part of the protuberance is detached, specifically, perpendicularly to the extent of the protuberance. This results, at the outer edge of the two casing halves, in each case in outwardly directed flanges. Via the flanges, the halves are connected to one another after fittings have been incorporated. Connection may take place, for example, by welding or adhesive bonding.
It also is contemplated however, not to cause the protuberance to run along the circumference, but along another line on the casing. The present invention further contemplates providing the protuberance in another suitable form which makes it possible to connect the two casing halves to one another in a simple way along the severing line after the at least partial detachment of the protuberance and incorporation of fittings.